1. Field of the Invention
The present invention relates to a power supply circuit having a first power supply as a single power supply and a second power supply as a single power supply.
2. Description of the Related Art
Some audio devices have a circuit operated by a first power supply as a single power supply and a circuit operated by a second power supply as a single power supply whose voltage is different from that of the first power supply. The single power supply includes a positive-side supply voltage and a reference potential such as a ground potential. The second power supply includes the positive-side supply voltage that is generated by switching and operating the positive-side supply voltage of the first power supply by means of a switching element, and a reference potential that is generated by dividing the positive-side supply voltage of the first power supply. A switching controller and the switching element are operated by the positive-side supply voltage and the reference potential of the first power supply. On the other hand, the positive-side supply voltage of the second power supply with respect to the reference potential should be fed back to the switching controller so as to be maintained at a predetermined voltage (for example, 5 V). However, since the first power supply and the second power supply have the different reference potentials, the positive-side supply voltage of the second power supply cannot be directly fed back to the switching controller. In a conventional power supply circuit, the positive-side supply voltage of the second power supply is fed back to the switching controller by using a photocoupler. However, since the photocoupler is comparatively expensive, there is demanded a power supply circuit that feeds back the positive-side supply voltage of the second power supply to the switching controller without using the photocoupler.
JP 2009-16997, JP 2009-194971, and JP 2009-303313 describe power supply circuits having a feedback circuit, but do not describe a power supply circuit having a first power supply as a single power supply and a second power supply as a single power supply whose voltage is different from that of the first power supply, in which the positive-side supply voltage of the second power supply is fed back to the switching controller.